Free piston devices use reciprocating pistons or displacers, but eschew crankshafts or other mechanical linkages to enforce motion, relying instead on the overall system dynamics as communicated by oscillating gas pressure to guarantee the proper motion amplitude and phase of different system components. As such, there is no natural constraint on piston mean position, which may drift off-center even if the dynamics are properly executed. This can result in shortening the useful stroke of the piston, or damage to the piston or other components if contact occurs.
In a reciprocating free piston device with clearance seals, the pistons will tend to drift toward the side with the greater oscillating pressure. This is understood to be due to the higher density of the gas flowing through the seal during the half-cycle where the high-oscillating pressure side is high, versus the reverse flow in the other half of the cycle. This results in a buildup of gas on one side of the piston, and a corresponding difference in mean pressure, which exerts a force on the piston thus causing it to move off-center.
Conventional corrective measures have fallen into three categories: (1) centerports, which are leak paths across the piston built into the piston seal so that they are open only when the pressure difference across the piston is supposed to be a minimum, and thus any built-up mean-pressure difference will tend to equalize twice in each cycle; (2) compensating flow networks which include networks with one-way valves connecting the two sides of the piston, or connected to reservoirs at different pressures, that are designed to leak fluid in a direction opposite to the unbalanced flow through the piston seal; and (3) perturbative DC currents where a DC current applies a corrective DC force to a free piston that is current-actuated (such as a linear motor) to oppose the pressure caused by fluid buildup on one side. The first two schemes involve a bypass path around the piston seal. The bypass path inevitably increases manufacturing costs, and introduces reliability and quality control problems. Many such schemes require some degree of active control and/or adjustment to work properly; centerports and check valves may become fouled with debris, and check valves may fail altogether. The bypass path also increases the dissipation in the system. A perturbative DC current requires additional sensors and control electronics, and is impractical on all but the smallest free piston devices.
Accordingly it is an object of this invention to provide an improved free piston device to better counteract piston drift.